The present invention relates to a laminated film and a packaging bag, and more particularly, to a laminated film which is prevented from being curled upon heat-sealing, and a packaging bag suitably used in a filling and packaging machine, which is produced by heat-sealing the laminated film.
Conventionally, various food and non-food products are packaged by a filling and packaging machine, and there have been used packaging bags produced by heat-sealing an unstretched laminated film. For example, in the case of packaging bags for foods such as hams and sausages, which are required to exhibit a gas-barrier property, there have been used such an unstretched laminated film comprising an outermost layer composed of polyethylene terephthalate (PET) or polyethylene naphthalate (PEN) having an excellent heat resistance, an innermost layer composed of a heat-sealing resin and an intermediate layer composed of a gas-barrier resin, or the like.
However, such conventional packaging bags tend to be curled at an open edge thereof, thereby causing problems when used in a filling and packaging machine. In addition, the unstretched laminated film itself tends to undergo considerable curling prior to being formed into a packaging bag by heat-sealing. For this reason, especially upon the production of pillow-type packaging bags, sealing defects tend to be caused during a so-called center seal process in which opposite edges of the unstretched laminated film are overlapped on each other and heat-sealed together.
As a result of the present inventors"" earnest studies for solving the above problems, it has been found that a laminated film produced by a co-extrusion inflation method, having an outermost layer composed of a polymer capable of satisfying specific parameters, is free from curling upon heat-sealing. The present invention has been attained on the basis of this finding.
It is an object of the present invention to provide a laminated film which is free from curling upon heat-sealing.
It is another object of the present invention to provide a packaging bag suitably used in a filling and packaging machine.
To accomplish the aims, in a first aspect of the present invention, there is provided a laminated film produced by a co-extrusion inflation method, comprising:
an outermost layer (A) comprising a polybutylene terephthalate homopolymer, a polybutylene terephthalate copolymer or a mixture of the polybutylene terephthalate homopolymer and the polybutylene terephthalate copolymer; and
an innermost layer (B) comprising a heat-sealing resin,
said outermost layer (A) having a shape factor of not less than 2.2, which shows a peak shape of Raman spectrum thereof and is represented by the following formula (1):
Shape factor=La/Lbxe2x80x83xe2x80x83(1) 
wherein La and Lb represent peak widths being present on higher- and lower-frequency sides, respectively, relative to a perpendicular line drawn from a peak top to a base line formed by a tangent line taken before and after the peak based on carbonyl stretching vibration, and being taken along a horizontal line at a height corresponding to one half of an intensity of the peak.
In a second aspect of the present invention, there is provided a packaging bag obtained by heat-sealing the laminated film as defined in the first aspect.
In a third aspect of the present invention, there is provided a food-packaging bag obtained by heat-sealing the laminated film as defined in the first aspect.
In a fourth aspect of the present invention, there is provided a laminated film produced by a co-extrusion inflation method, comprising:
an outermost layer (A) comprising a polybutylene terephthalate homopolymer, a polybutylene terephthalate copolymer or a mixture of the polybutylene terephthalate homopolymer and the polybutylene terephthalate copolymer;
an adhesive resin layer comprising a modified polyolefin resin; and
an innermost layer (B) comprising a heat-sealing resin,
said outermost layer (A) having a shape factor of not less than 2.2, which shows a peak shape of Raman spectrum thereof and is represented by the following formula (1):
Shape factor=La/Lbxe2x80x83xe2x80x83(1) 
wherein La and Lb represent peak widths being present on higher- and lower-frequency sides, respectively, relative to a perpendicular line drawn from a peak top to a base line formed by a tangent line taken before and after the peak based on carbonyl stretching vibration, and being taken along a horizontal line at a height corresponding to one half of an intensity of the peak.
In a fifth aspect of the present invention, there is provided a laminated film produced by a co-extrusion inflation method, comprising:
an outermost layer (A) comprising a polybutylene terephthalate homopolymer, a polybutylene terephthalate copolymer or a mixture of the polybutylene terephthalate homopolymer and the polybutylene terephthalate copolymer;
a first adhesive resin layer comprising a modified polyolefin resin;
a gas-barrier resin layer (C) comprising polyamide, an ethylene-vinyl acetate copolymer saponification product, polyethylene terephthalate, polyethylene naphthalate or polycarbonate;
a second adhesive resin layer comprising a modified polyolefin resin; and
an innermost layer (B) comprising a heat-sealing resin,
said outermost layer (A) having a shape factor of not less than 2.2, which shows a peak shape of Raman spectrum thereof and is represented by the following formula (1):
Shape factor=La/Lbxe2x80x83xe2x80x83(1) 
wherein La and Lb represent peak widths being present on higher- and lower-frequency sides, respectively, relative to a perpendicular line drawn from a peak top to a base line formed by a tangent line taken before and after the peak based on carbonyl stretching vibration, and being taken along a horizontal line at a height corresponding to one half of an intensity of the peak.